The present invention relates to a small metal halide lamp having a rated power of not more than 100 W and, more particularly, to a small metal halide lamp which emits light having a similar color tone and similar spectral characteristics to those of an incandescent lamp.
From the viewpoint of energy consumption, there has been great demand recently for a small metal halide lamp having a high luminous efficacy and high color rendering properties, to replace incandescent lamps which have been widely used as indoor light sources in stores and in the home.
Medium and large size metal halide lamps of a power greater than 100 W are already known and used. Among these metal halide lamps, the large metal halide lamp has a luminous flux value significantly greater than that of the incandescent lamp, so that it is installed at a relatively higher position to effectively utilize this amount of light even if it is used indoors where high color rendering properties are required. Although metal halide lamps have both a high luminous flux value and high color rendering properties, they do not often receive much attention. In order to use a metal halide lamp in place of the incandescent lamp, an object must be directly irradiated in the same manner as with an incandescent lamp so as to emphasize the color tone of the object. For this purpose, the color rendering properties of the metal halide lamp are very important in providing warm color lighting indoors (based on elements such as the color tone of light and the color temperature), and in eliminating any disharmony between a metal halide lamp and an incandescent lamp which may be used together as light sources.
The color temperature of the metal halide lamp is preferably as low as 3,000 K, as compared with the color temperature of the incandescent lamp. Furthermore, the chromaticity of the metal halide lamp must not greatly deviate from the black body locus (to be referred to as a BBL hereinafter). The high luminous efficacy of the metal halide lamp must also be retained from the viewpoint of low power consumption.
In a metal halide lamp, the type of halide to be contained in an arc tube largely determines various characteristics such as the color temperature, luminous efficacy, and color rendering properties. Especially, among the conventional halides, sodium halide and scandium halide are suitable as halides which provide a low color temperature, a high luminous efficacy and high color rendering properties, in accordance with studies made in the development of the large metal halide lamp. However, when the techniques used for manufacturing the large metal halide lamps are used for manufacturing a small metal halide lamp having a rated power of 100 W or less, various problems are presented.
One of the problems is degradation in luminous efficacy of the lamp. When the lamp size is decreased, its luminous efficacy is generally degraded. The following causes for the degradation in luminous efficacy are considered: circulation of metal vapor cannot be smoothly performed since the discharge space is decreased; and since the sealed portion is increased with respect to the discharge space and the heat loss from the sealed portion is increased, the temperature of the coldest spot cannot be increased, thereby decreasing evaporation of the contained metal.
In order to eliminate the above problems, the arc tube is formed to have a spheroidal or ellipsoidal shape so as to accelerate the circulation of gas in the discharge space. Furthermore, the sectional area of the sealed portion is decreased to prevent heat loss, thereby increasing the temperature of the spot of the coldest temperature. Alternatively, a tube wall load is increased as compared with that of the medium and large metal halide lamps. In the small metal halide lamp which has sodium halide and scandium halide and which is treated to prevent degradation in luminous efficacy, its color temperature is decreased by about 500 to 600 K as compared with a color temperature of 4,000 K of a metal halide lamp of 400 W. Furthermore, the color rendering properties of the small metal halide lamp of the type described above are slightly improved. This is because the light-emitting intensity of the contained material must be increased to compensate for the heat loss when the size of the metal halide lamp is decreased. This improvement is preferable to achieve the color temperature and color rendering properties of the metal halide lamp which resemble those of the incandescent lamp. However, in the small metal halide lamp which provides a high luminous efficacy, high color rendering properties, and a low color temperature, the chromaticity is greatly deviated from the BBL. The color tone of light becomes pinkish or of red purple due to an increase in light emission from sodium, resulting in a great difference from the color of light from the incandescent lamp. In this manner, when the color of light from the metal halide lamp differs greatly from that of light from the incandescent lamp, disharmony between these colors is presented. As a result, warm color lighting and comfort, which are requirements for indoor lighting, are impaired.
The present inventors have made extensive studies on the small metal halide lamp of the type described above so as to improve the color tone of light therefrom. It is found that a phosphor coated on the inner surface of an envelope improves the color of light emitted therefrom to eliminate disharmony between the small metal halide lamp and the incandescent lamp. The essential object of the present invention is to improve the color tone of light emitted from the lamp by coating a phosphor on the inner surface of the envelope.
The technique of applying a phosphor on the inner surface of the envelope to substantially equalize the spectral characteristics of a metal halide lamp with those of an incandescent lamp is described in Japanese Patent Disclosure No. 52-135,581 (to be referred to as the prior art hereinafter). In the technique described in the prior art, the objective is the manufacture of medium and large metal halide lamps having a rated power of 400 W. Therefore, the prior art differs from the present invention in which a metal halide lamp of 100 W or less is an essential objective.
In a chromaticity diagram shown in FIG. 1, the spectral characteristics of the arc tube of the prior art are distributed on or above the BBL, as the color tone of light emitted from the arc tube is indicated by a circle. When a red and green phosphor is coated on the inner surface of the envelope, the spectral characteristics can be improved as indicated by arrows A and B. Specifically, when the red phosphor is used, the color temperature is changed as indicated by arrow A along the X-axis on the X-Y coordinates. The color temperature can be decreased to about 3,000 K. However, when the color temperature is decreased to about 3,000 K using the red phosphor, the spectral characteristics are greatly deviated from the BBL. In order to compensate for this deviation, the green phosphor is used to redistribute the circles along the Y-axis so as to obtain the spectral characteristics which resemble those of the incandescent lamp.
In the small metal halide lamp of 100 W or less according to the present invention, it is found that the color tone of light emitted from the arc tube which is treated to prevent degradation in luminous efficacy, color rendering properties and color temperature is distributed as indicated by a square. The position of the square is lower than that of the BBL, but the color temperature is considerably low. The color tone of light from the metal halide lamp of the type described above can be converted such that the squares are moved in the direction indicated by arrow C. The color temperature need not be decreased but the chromaticity should be increased along the Y axis to come close to the BBL.
When the prior art is applied to the small metal halide lamp according to the present invention, the color temperature is decreased too much to move squares in the direction parallel to the direction indicated by arrow A, so that the squares are greatly deviated from the BBL. Therefore, the squares must be moved upward along the Y-axis using the green phosphor. However, it is impossible to correct such a great deviation as described using the conventional green phosphor. A phosphor which absorbs blue light is thus required.
As described above, according to the prior art, the chromaticity of light emitted from the medium and large metal halide lamps, that is, from the arc tubes of the lamps, is distributed above the BBL. The technique of the prior art is effective when the light is distributed on or slightly below the BBL. However, the prior art cannot be applied to the small metal halide whose chromaticity has a deviation of 0.010 UV from the BBL.